Multi-mode lasers having outputs with a large number of frequency or spectral components called mode lines have been utilized in the past for a variety of applications. Whether these lasers are mode locked or not, the existence of multiple modes results in output pulses from the lasers having discrete or distinct spectra at closely spaced frequencies surrounding the spectral line to which the laser is originally tuned. While in most applications production of a comb-like spectral response is not only important but is often times sought, in some applications it is desirable to displace or shift the spectra of the output signal from the laser during the production of a laser pulse so that the output signal appears to have a continuous frequency spectrum to either side of the frequency to which the laser is tuned. In this technique, the mode lines are in effect "smeared out" to give the pulse an energy vs. frequency continuum.
This smearing out of the spectra in multi-mode lasers is important in certain types of isotope separation processes in order to achieve maximal efficiency. One such process is described in U.S. Pat. No. 3,772,519 for a Method and Apparatus for the Separation of Isotopes by R. H. Levy et al. In this patent a method is disclosed for isotope separation in which an environment containing a plurality of uranium isotopes is radiated with laser radiation of a particular frequency to selectively excite the particles of the desired isotope type. When certain particles are selectively excited, the selectively excited particles may be separated as disclosed in the patent. For optimum efficiency, the laster isotope separation process, as shown in the Levy et al patent, prefers excitation radiation with energy distributed throughout the bandwidth of the absorption structure of the U-235 component of uranium vapor rather than the series of discrete mode frequencies typical of most laser radiation.
The subject invention also has application to atomic and molecular systems in which the exact position of the spectral components or transitions cannot easily be either calculated or verified experimentally. Uncertainty in spectral line position may occur through Doppler broadening, Zeeman shift or cross-coupling interactions not thoroughly understood.
In order to insure that optimal energy is available to provide a predetermined spectral interaction, it is oftentimes desirable that the spectral response of the laser be slightly broadened so as to accommodate either known or unknown shifts in the corresponding spectral line.
In so doing multi-mode lasers may be given a virtually continuous frequency profile regardless of the modal nature of the output.
One way of achieving the even energy distribution is to "chirp" or sweep the frequency of the laser radiation, thereby moving the spectral line. In the isotope separation case this results in all portions of, for instance, the U-235 absorption band, being subject to resonant frequency laser radiation. Representative methods of causing such a chirp are shown in U.S. Pat. No. 3,611,182, issued to E. B. Treacy, involving the utilization of a rotating mirror, and U.S. Pat. No. 4,088,898, issued to M. L. Stitch which utilizes a rotating optical wedge which varies in thickness and presents the varying thickness to the optical path of the laser during the generation of a laser pulse. In this patent the optical wedge has an axis of rotation parallel to the input beam. The latter of these two patents is assigned to the assignee of the present application.
Additionally, broadening of the spectral response of multi-mode lasers is elaborated upon in copending U.S. patent application Ser. No. 862,409 filed Dec. 29, 1977 by Hans A. Bethe and Ching Sung Chang, the application also having been assigned to the assignee of this application.
With respect to the Treacy patent it will be appreciated that while the rotation of the mirror at the end of and optical cavity may vary the cavity length, it also deflects away a considerable portion of the radiation which is to be transmitted through the cavity. For systems which require considerable energy density, the deflection away of energy reduces the efficiency of the system.
With respect to the Stitch patent, this system operates satisfactorily for the purposes intended and is especially useful in isotope separation. It will be appreciated, however, that forming the optical wedge with a contoured surface requires complex optical glass working. Moreover, utilization of a large unbalanced mass may in some extreme cases produce shaft misalignment and vibration in which the beam may be deflected off axis as it passes through the wedge. Additionally, due to the configuration of the rotating wedge there is a possibility of introducing a lens effect (e.g. unwanted focusing) on the beam which passes through the wedge. This occurs when light impinges on the entire face of the wedge. In the Stitch patent, beam reducing and beam extending optics are provided so that the input beam only impinges on a small portion of the surface of the wedge.